Amiodarone induced thyroid dysfunction: A high cumulative incidence in a nationwide cohort study in Iceland.

Background: Amiodarone induced thyroid dysfunction (AITD) is divided into amiodarone induced thyrotoxicosis (AIT) and amiodarone induced hypothyroidism (AIH). The prevalence of them varies from 1.2% to 12% for AIT and 12%-17% for AIH.

Objectives: To study the incidence and complications of AITD.

Methods: The cohort comprised all euthyroid patients who filled their first amiodarone prescription in Iceland in 2014, 262 persons. Data were gathered with chart review, and diagnosis confirmed with thyroid function tests. The cumulative incidence accounting for death as a competing risk was estimated for AIT, AIH, and AITD with three separate Fine-Gray models.

Results: The overall incidence of AIT, AIH, and AITD was 9.2% (95% CI: 5.6%-12.7%), 13.4% (95% CI: 9.2%-17.5%), and 22.5% (95% CI: 17.4%-27.6%), respectively, and the 5-year cumulative incidence in the same order was 19.0% (95% CI: 11.9%-25.5%), 21.8% (95% CI: 14.7%-28.2%), and 38.5% (95% CI: 30.4%-45.7%). The highest yearly incidence rate of AIT was 9.8% during the third treatment year, and for AIH, it was 9.8% during the first year of treatment. The complications of AIT were hypothyroidism (8%), thyroidectomy (8%), hospitalizations (36%), and death (4%). Most patients (91.7%) with AIH were placed on thyroid replacement therapy.

Discussion: Nearly 40% of patients taking amiodarone for 5 years acquire thyroid dysfunction, which is higher than previously described. Frequent monitoring of thyroid function should be considered during the high-risk periods of the first and third treatment years.